Full configuration interaction calibration

The full configuration-interaction (FCI) method [8], in particular, occupies a uniquely important position in ab initio MO theory to this day, as it provides the exact basis-set solutions of the Schrbdinger equations of general polyatomic molecules [9-26]. These solutions serve as essential benchmark against which approximate ab initio methods are calibrated and tested [27-38]. 

The time-independent Schrddinger equation within the limits of a particular basis set can be solved exactly by Cl if the Cl space is not truncated. Such full configuration interaction (FCI) studies of methylene employing several basis sets of double- (DZ) quality calibrate results for A sx as well as for the geometries.Although these A sx values are more than 3 kcal mol" above the firmly established experimental value,the exact solution of the Schrodinger equation in the space spanned by the basis set allows for a systematic comparison of the performance of various computational methods. The results presented in Table 1 were obtained at selected levels of theory employing two different DZ basis sets. One of them (double- plus polarization, DZP) uses the same, the other (DZP ) uses different orbital exponents for the C and H polarization functions for the B and A) states. 

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